1. Field of the Invention
The present invention relates to inertial navigation systems. More particularly, this invention pertains to a method and apparatus for reducing the effects of gyro drift rate, gyro scale factor errors, gyro-sensing orientation uncertainties and accelerometer bias errors for a set of inertial instruments.
2. Description of the Prior Art
Inertial navigation systems are commonly utilized for determining the position of a vehicle relative to a coordinate system that is fixed relative to the earth which in turn rotates with respect to inertial space (the stars) located at a reference point such as the center of the earth. The computed position of the vehicle derived from the inertial instruments (gyros and accelerometers) is normally used to compute commands for controlling the vehicle.
Autonomous inertial navigation systems generally determine the position by means of on-board instruments which measure acceleration and angular rotation rate with respect to inertial space. Such navigation systems are particularly desirable for vehicles such as submarines which remain submerged for extended periods of time. Autonomous inertial navigation systems generally measure acceleration along each of three axes of an orthogonal coordinate system. After an initial alignment, the directions of these axes are determined by measuring their rotation with respect to inertial space by a triad of gyroscopes. The velocity and position of the vehicle are normally determined by transforming the accelerometer measurements to a convenient orthogonal coordinate frame whose axes are related to the earth (e.g., east, north and vertical axes), correcting for Coriolis acceleration components and the force of gravity and integrating once to obtain velocity and again to obtain position, with the aid of a computer.
When a triad (or more) of gyroscopes is rotated with respect to inertial space about a single axis, the gyroscope outputs generally include three kinds of drift errors: gyro drift rate, gyro scale factor error, and gyro misalignment (or sensing axes orientation uncertainty). As a result of such inherent errors, the directions of the measured accelerations, determined from the gyroscope outputs, produce a difference between the navigated and the actual or "true" position of the vehicle. Over the long term, such a difference may increase substantially and can prove to be dramatic.